1. Field of the Invention
The present invention relates to an image processing system, and in particular relates to an image processing system applicable to three-dimensional computer graphics in a workstation, game equipment and so forth which can in real time output an image signal based on image information concerning polygons.
2. Related Art
An image processing system in the related art is one which outputs various kinds of image signals after composing them for displaying on a CRT, based on image information supplied from outside the image processing system.
Such a system may output not only a two-dimensional plane image but also a stereoscopic two-dimensional image, that is, a simulated three-dimensional image. Thus, such a system may be widely used for a game equipment for three-dimensional images, computer graphics, and so on, for example.
A bit-mapped display (graphic display) method may be used for such a kind of image processing system. There, a bit-mapped memory is provided having storing areas respectively corresponding to all pixels, one by one, constituting a relevant CRT screen image. All pixel information needed to display one image on the CRT screen is stored in each storing area of this memory. In one example of such a kind of image processing in computer graphics, in a case where an arbitrary figure composed of polygons is to be displayed, the following procedures are repeatedly executed: A contour of a polygon is drawn on the screen, and then the interior thereof is painted with a specified color stored in the memory. Therefore, a large number of polygons requires a large-capacity memory and may not allow real-time processing.
Japanese Laid-Open Patent Application No. 62-231379 (international patent classification: G06F 15/62) discloses an image composing apparatus. This apparatus can in real time output an image signal based on information concerning a contour of a specified figure. This apparatus comprises contour-point information storing means for storing contour lines associated with a figure to be displayed on a CRT in a horizontal-scan line storing area provided corresponding to each horizontal scan line. There, the contour lines to be thus stored are successively written in the area according to their priority levels. The apparatus further comprises an index memory. An accompanying data packet included in each contour information packet from the horizontal-scan storing area corresponding to the relevant vertical scanning position is then read. The above index memory is used for storing the thus read accompanying data packets at the index-memory addresses corresponding to their priority levels. The above reading and storing operation is executed in synchronization with the horizontal scanning signal. The apparatus further comprises an address generating means. In synchronization with the horizontal scanning signal, the address generating means successively reads a contour-point pair included in each contour information packet from the horizontal-scan storing area corresponding to the relevant vertical scanning position. Then, the address generating means, when the horizontal scan is being performed in a region specified by each contour-point pair, outputs, to the index memory, the reading address concerning the corresponding accompanying data based on its priority level. Thus, the apparatus in real time outputs the image signal for the display on the CRT based on the contour-point information concerning the specified figure.
The above-described image composing apparatus comprises a field processor for successively processing the polygons in a digital differentiation analysis (DDA). The apparatus stores the result of the above operation into a frame memory. It is necessary to store, in this frame memory, accompanying information for indicating X-starting points and X-ending points associated with the polygons on one scan line, and their attributes, after all polygons are processed. Therefore, a large-capacity memory is required for the frame memory, which has a high cost. In particular, an increase in the amount of the accompanying information concerning the polygons results in a sharp increase in the required memory capacity. Thus the memory portion occupied in the apparatus becomes extremely large.